<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>获得元素</title>
</head>
<body>
    <button>确定</button>
    用户名：<input type="text" class="username">
    密码：<input type="password" class="password"><br>
    <script src="js/js_02.js"></script>
</body>
</html>